1. Field of the Invention
This invention relates to a developer for developing electrostatic images by image forming methods such as electrophotography, electrostatic recording, and electrostatic printing. More particularly, the present invention pertains to a positively chargeable one component magnetic developer which is charged uniformly positively in the direct or indirect electrophotographic developing method to give an image of high quality by visualizing negative electrostatic images or visualizing positive electrostatic images by reversal development.
2. Related Background Art
In the prior art, there have been known as the electrophotographic methods such those as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (U.S. Pat. No. 3,666,363), and Japanese Patent Publication No. 43-24748 (U.S. Pat. No. 4,071,361). Generally speaking, a photoconductive material is utilized to form electrostatic latent images on a photosensitive member by various means, then said latent images are developed by use of a toner or a developer, and after transferring optionally the toner images on a transfer material such as paper if necessary, the images are fixed by heating, pressure, pressure and heat fixing roller or solvent vapor to obtain a copied product. When transferring the toner image, there is provided generally the step for removing the residual toner on the photosensitive member.
As a development method for visualizing electrostatic latent images by use of a toner, there have been known the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063, the cascade developing method disclosed in U.S. Pat. No. 2,618,552 and the powder cloud method disclosed in U.S. Pat. No. 2,221,776, and the method in which an electroconductive magnetic toner is used as disclosed in U.S. Pat. No. 3,909,258.
As the toner to be applied for these developing methods, fine powder containing a dye, a pigment dispersed in a natural or synthetic resin has been employed. For example, particles having a dispersion of a colorant in a binder resin such as polystyrene which are pulverized finely to about 1 to 30.mu. have been used as the toner. As the magnetic toner, those having magnetic particles such as magnetite contained therein have been used. In the case of a system employing the two component system developer, the toner is generally employed as a mixture with carrier particles such as glass beads, iron powder, ferrite powder.
As the positively charged controlling agent to be used for the toner for such dry process development, for example, a quaternary ammonium compound, an organic dye (especially basic dye or its salt), Nigrosine base or Nigrosine has been employed. These are generally added in a thermoplastic resin, dispersed by melting under heating, and the mixture is finely pulverized and controlled to suitable particle sizes, if necessary.
However, these charge controlling agents are liable to cause the phenomenon of lowering in charge controllability due to mechanical shock, friction, changes in temperature and humidity conditions.
Accordingly, when development is performed by use of a toner containing these as the charge controlling agent in a copying machine, deterioration of the toner may be sometimes caused to occur during successive copying as the number of copies increases. Since these charge controlling agents are dispersed uniformly in a thermoplastic resin with extreme difficulty, there is the problem that there is readily caused a difference in triboelectric charges between the toner particles obtained by pulverization. For this reason, various methods have been practiced in the prior art for effecting dispersion more uniformly. For example, a basic Nigrosine dye is used in the form of a salt formed with a higher fatty acid in order to improve compatibility with a thermoplastic resin. However, unreacted fatty acid or the dispersed product of salt is frequently exposed on the toner surface to cause contamination of the carrier or the toner carrying member, lowering in flowability of the toner, fogging, or lowering in image density. For the purpose of improving dispersion of these charge controlling agents into the resin, there is also employed the method in which the charge controlling agent powder and the resin powder is mechanically pulverized and mixed before hot melting and kneading. However, the inherent dispersion difficulty cannot be avoided, and it has been desired to have a developer having more uniform positive chargeability.
There has been also an attempt to give uniform charges to the toner by making the binder resin itself positively chargeable by introducing amino groups by way of copolymerization or graft polymerization of a positively chargeable monomer such as dimethylaminoethyl methacrylate into a binder resin. However, the positive chargeability of the binder resin as mentioned above is not necessarily constant, but it is greatly changed depending on the magnitude of frictional force and friction probability between the toner particles, or between toner and carrier, and between toner and toner carrying member such as sleeve, and therefore it is not easy to provide always constant and stable positive charges to the toner. Accordingly, the positive chargeability of the toner when no adequate friction can be obtained is very unstable, whereby the copied image obtained by said toner becomes an image with much fog and scattering. On the contrary, when excessive friction is caused, the positive charges on the toner surface become extremely great, such that only an image with excessive coarseness and low density can be obtained.
Japanese Patent Laid-Open Application No. 59-201063 (U.S. Pat. No. 4,568,625) discloses a technique in which a positively chargeable developer is prepared by use of a silica fine powder treated with a silicone oil having amines in the side chain. Japanese Patent Laid-Open Application No. 61-160760 (U.S. Pat. No. 4,666,813) proposes a technique in which a specific fluorine-containing compound is added in the developer.
The present inventors have found that in a digital copying machine which develops digital latent images with low potential contrast of latent image (e.g. 300 V or less) according to the reversal developing system, the image density tends to be lowered in continuous successive copying test of a large number of sheets with the use of a developer in which a positively chargeable silica is merely mixed within the positively chargeable toner. The tendency is also the same in a developer in which an external agent such as polyvinylidene fluoride fine particles is added in the positively chargeable toner and further, a developer comprising a mere mixture of a positively chargeable toner, silica fine particles and polyvinylidene fluoride fine particles is still unsatisfactory with respect to developing characteristics and durability in the light of the present desire for higher image quality and higher durability.
On the other hand, in a high speed copying machine which effects normal development of negatively charged electrostatic latent images at a process speed of 300 mm/sec or higher, a high performance positively chargeable one component developer has been also awaited.
As a specific example, although only a negatively chargeable developer is disclosed, Japanese Patent Laid-Open Application No. 61-250658 proposes a developer containing fine particles of the same polarity to the charged polarity and fine particles of the opposite polarity thereto. The present inventors prepared a developer by imparting positively chargeable silica and negatively chargeable silica to a positively chargeable toner, and conducted successive copying test by means of the digital or high speed copying machines. Only insufficient developability could be obtained.
A-Si (amorphous silicon) has high photosensitivity over the whole visible region, and therefore can correspond to a copying machine or printer by use of a semiconductor laser or a copying machine for color. A-Si photosensitive drum has a high surface hardness, can be expected to have long life, having a Vickers hardness of 1,500 to 2,000, and also has successive copying performance of 20 to 500,000 sheets which is several-fold of the CdS photosensitive member which is said to have the highest durability and abrasion resistance. Even with respect to heat resistance, it can be sufficiently used within the range of the practical level of an electronic copying machine.
In spite of such advantages, amorphous silicon involves the problems in reduction of cost and productivity. Generally speaking, the surface dark potential of A-Si photosensitive member corresponding to film thickness is said to be 20 to 30 V/.mu.m. The surface dark potential of the photosensitive member practically applied at present is required to be 500 V at the minimum in CdS system, and 600 to 800 V in the Se system and the organic photoconductive (OPC) member system. For accomplishing this potential in A-Si, at least a film thickness of 30.mu. or more is necessary. In view of various fluctuations of characteristics, lowering in sensitivity depending on the difference of environment, A-Si should preferably have a film thickness of 40.mu. or more. For obtaining a film thickness of 40.mu. or more, there ensue the problems of elevation in production cost and lowering in production capacity of A-Si. Increase of film thickness tends to cause abnormal growth of A-Si film during preparation steps, whereby partially non-uniform A-Si film is formed to give rise to irregularity in the image, making the film practically useless. To cope with such problems, it has been proposed to make the film thickness of A-Si as thin as 5 to 25.mu., while satisfying both aspects of productivity and cost, and performances of A-Si photosensitive member. When the A-Si film thickness becomes 5 to 20.mu., the surface dark potential which can be used stably becomes 300 to 400 V. In such case, it is extremely difficult to obtain stably sufficient solid black at a low potential with a developing contrast between the light portion and the dark portion of 300 V or less (e.g. 280 to 250 V) with a conventional developer. The developing contrast in normal developing refers to the absolute value of the average dark portion potential of the photosensitive drum from which the developing potential was substrated.
In order that an A-Si photosensitive member which has been made thinner can be practically used under such conditions, a toner having uniform and high charging ability capable of developing at low potential must be used.
Particularly, when the image signals are digital signals, the latent images are formed by gathering of the dots of constant potential, and the solid portion, the half tone portion and the light portion are respectively expressed by changing the density of the dots. Accordingly, every portion is formed of electrostatic latent images of basically the same potential in the case of binary value.
There has been also developed the multi-value recording method, in which information in the depth direction per one dot is given from the binary value method of the prior art as described above.
The method, in forming images by a laser beam printer by forming digital signal values into binary values, in order to obtain gradation characteristics of half tone, comprises converting the digital image signals into analog signals, generating binary value signals applied with pulse width modulation by comparing the analog signals with periodical pattern signals such as triangular waves, and utilizing the binary signals as the driving signals for the laser beam source. Thus, by pulse width modulation of the digital image signals, it becomes possible to obtain both high resolution and high gradation characteristics.
However, when a negatively charged electrostatic latent image on the A-Si drum formed by the digital signals as described above is reversely developed by use of a positively chargeable developer of the prior art, a number of problems occur because of non-uniform charging generated on the toner particle surfaces.
When the developing contrast of the latent image potential is low, and if developing is repeated, toner particles having uniform charging are preferentially consumed for development (the so-called selective development), to give the result that when continuous copying is conducted, the ratio of the toner particles non-uniformly charged will be increased to bring about various problems such as lowering in image density, and lowering in image quality.
Recently, an OPC drum for forming negatively charged latent images has been attempted to be made durable and a positively chargeable toner has been applied for high speed machines. In this case, not only in development of digital latent images as described above, but also in development of analog latent images, it is required to have a positively chargeable developer having high durability which can stand copying of more sheets than in the prior art.
Further, ground fog, reversal fog, and coarseness tend to be worsened in direct proportion to the increase in process speed, and particularly the tendency is marked in reversal fogging. This phenomenon may be estimated to be caused by the fact that the sliding between the toner and the toner carrying member becomes smaller in chance and also shorter with the increase of the process speed, whereby the toner cannot obtain sufficient and uniform charging.